Low-power feedforward amplifier

ABSTRACT

A feedforward trunk amplifier having the distortion advantages associated with the feedforward topology while providing significantly reduced power consumption and cost. The trunk amplifier has a discrete preamplifier including active gain and slope control, and a low-power feedforward output stage including a medium-gain, hybrid main amplifier, a high-gain, discrete error amplifier having two gain stages which share the same bias current, and miniature coaxial cable delay lines mounted on the cover of the amplifier housing.

This application is a continuation of application Ser. No. 833,651,filed Dec. 8, 1986, now U.S. Pat. No. 4,677,390 issued June 30, 1987.

BACKGROUND OF THE INVENTION

This invention relates to low-distortion amplifiers, and particularly tofeedforward amplifiers.

Feedforward is a well-known technique for reducing the effects ofdistortion and noise products generated by an amplifier. A sample of theamplifier's output signal is compared with a sample of its input signalto produce an error signal proportional to the internally generateddistortion and noise products, and the error signal is amplified andcombined with the amplifier's output signal in opposite phase tosubstantially cancel the distortion and noise products.

Feedforward is particularly useful in signal transmission systemsemploying coaxial lines, where amplifiers are often connected in cascadefor periodic signal boosting to overcome line attenuation. Cascading ofamplifiers is routine in CATV systems, for example, where television andother signals are transmitted from a central transmitting station, knownas the head end, via a coaxial line network having one or more trunklines and a number of feeder lines connected to each trunk line. Trunkamplifiers are spaced at appropriate intervals along each trunk line toamplify signals being transmitted through the system, and bridgingamplifiers are connected to trunk amplifiers to provide multiplehigh-level outputs for driving feeder lines. Subscriber taps are coupledto the feeder lines, which typically also include line extenderamplifiers which boost the transmitted signals and thereby extend thefeeder lines. Many systems also include a return trunk for two-waytransmission. A feedforward topology reduces distortion productsgenerated in the process of signal amplification and thereby allowsoperation at higher signal levels at the output of any one amplifierstation with an acceptable level of distortion, thus facilitatingbandwidth expansion, i.e., carriage of more channels without incurringsignificant degradation in distortion performance. The higher outputlevels are required to overcome the increased cable attenuation athigher frequencies.

A 20 dB reduction in third order distortions such as composite triplebeat and crossmodulation effects can be achieved with feedforwardtopologies, but with power consumption three times that of a hybridamplifier, the conventional gain block in broadband CATV distributionsystems prior to the use of feedforward. The localized threefoldincrease in power associated with the feedforward device requires acopper heat spreader to distribute the heat throughout the station. Anadequate heat spreader occupies a substantial amount of space in anamplifier station and, due to labor-intensive machining, is an expensiveitem. Without considering heat spreader costs, a feedforward devicealready costs approximately ten times as much as a conventional hybridmodule. Because of these and other limitations, the heat spreaderconcept seriously impairs upgradability, via field retrofits, toexisting stations. In some instances, field upgrading of trunk stationsto feedforward requires removal of all modules, including, as will bedescribed later, forward, reverse and bridger amplifiers and AGC, sothat the heat spreader may be installed in the station. In addition tothe inconvenience to the technician in installing the new equipment,there would be an undesired interruption in service along the returnsegment of the trunk. Some systems employ telemetry and security systemsalong the return trunk, and a breach of security is unacceptable.Moreover, although a heat spreader allows feedforward in a trunkamplifier module, it is ineffective at heat dispersion when feedforwardis also used in the bridger module, where it is required for systemupgrades in bandwidth.

Another problem with current feedforward amplifiers involves high-gainhybrids, that is, hybrids having gain of 34-38 dB, which are commonlyused as building blocks for such amplifiers, as will be described laterin more detail. High-gain hybrids have frequency response problems whichhybrid manufacturers have not adequately resolved for devices operatingfrom a negative power supply. Since adequate negative-supply feedforwarddevices are lacking, existing product lines using a negative powersupply cannot be upgraded to feedforward topology without installationof a dual-polarity power supply in the station to operatepositive-supply feedforward devices along with the negative-supplyconventional devices utilized in the other amplifier modules. Thiscauses a serious impact in station space availability and increasescost.

SUMMARY OF THE INVENTION

The present invention provides an improved amplifier having thedistortion advantages of feedforward and the low power consumption ofconventional hybrid amplifiers.

A low-power feedforward amplifier according to one aspect of theinvention comprises a main amplifier and means for comparing a sample ofan input signal supplied to the main amplifier with a sample of the mainamplifier's output signal, the comparing means producing an error signalproportional to the difference between the signal samples. The amplifierfurther comprises means for amplifying the error signal and combiningthe amplified error signal with the main amplifier output signal inopposite phase, whereby distortion and noise products generated by themain amplifier are substantially cancelled. The amplifying and combiningmeans includes an error amplifier having a plurality of gain stageswhich share the same bias current.

According to an aspect of the invention particularly applicable to CATVsystems, a low-power feedforward amplifier is provided which comprisesan active gain and slope control network operative in conjunction with afeedforward stage, the feedforward stage including a main amplifier,means for comparing a sample of an input signal supplied to the mainamplifier with a sample of the main amplifier's output signal, thecomparing means producing an error signal proportional to the differencebetween the signal samples, and means for amplifying the error signaland combining the amplified error signal with the main amplifier outputsignal in opposite phase, whereby distortion and noise productsgenerated by the main amplifier are substantially cancelled.

A general object of the present invention is to provide an improvedfeedforward amplifier.

Another object is to provide a feedforward amplifier having reducedpower consumption.

A further object of the invention is to reduce interstage losses in amultistage trunk amplifier module.

These and other objects and advantages of the present invention willbecome more apparent upon reference to the following detaileddescription and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a trunk amplifier station of the type usedin a CATV system.

FIG. 2 is a drawing partly in schematic form and partly in block diagramform of a conventional forward amplifier for use in the trunk amplifierstation shown in FIG. 1.

FIG. 3 is a drawing partly in schematic form and partly in block diagramform of the preferred embodiment of a forward amplifier according to thepresent invention.

FIGS. 4A-4E depict an active gain and slope control network according tothe present invention, along with related operating curves.

FIG. 5 is a block diagram of a prior art feedforward amplifier.

FIG. 6 is a block diagram of a low-power feedforward amplifier employedaccording to the preferred embodiment of the present invention as theoutput stage of an amplifier such as shown in FIG. 3.

FIG. 7 is a schematic diagram illustrating the current sharing featureof the preferred embodiment of the invention.

FIG. 8 is a diagram of a bottom cover of an amplifier housing showingthe position of delay lines according to one embodiment of theinvention.

FIG. 9 is a block diagram of a low power feedforward amplifier employedaccording to the preferred embodiment of the present invention as theoutput stage of an amplifier such as shown in FIG. 3.

FIG. 10 is a schematic diagram illustrating current sharing betweenisolated amplifiers.

FIG. 11 is a schematic diagram depicting an alternative embodiment of anactive gain and slope control network according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

The preferred embodiment of the present invention is a CATV feedforwardamplifier, specifically a feedforward amplifier used as an output stagein a multistage CATV amplifier. However, the principles of the inventionmay be advantageously applied to amplifiers, including single-stageamplifiers, for use in other types of systems where low distortion andlow power consumption are desired.

As stated previously, television and other signals are transmitted fromthe head end of a CATV system via a coaxial line network having one ormore trunk lines which each have a number of associated feeder lines.The signals carried by each trunk are periodically amplified by trunkamplifiers to overcome cable attenuation. Trunk amplifier spacing isthus a function of cable attenuation and, accordingly, is customarilyspecified in dB to account for different losses per unit lengthassociated with different types of cable. Bridging amplifiers connectedto the trunk amplifiers provide multiple high-level outputs for drivingfeeder lines, which typically include line extender amplifiers.

A trunk station according to the preferred embodiment of the inventionis shown in block diagram form in FIG. 1 wherein it is representedgenerally by reference numeral 10. Trunk station 10 is a two-way CATVtrunk amplifier unit connected to the trunk by lines 12 and 14 on theupstream (head end) and downstream sides of the station, respectively.Trunk signals from the head end are amplified and conducted downstreamthrough line 12, diplex filter 16, forward amplifier 18, directionalcoupler 20, diplex filter 22 and line 14. Trunk signals from downstreamsources are amplified and conducted upstream through line 14, diplexfilter 22, reverse amplifier 24, diplex filter 16, and line 12. Thetrunk station includes a bridger amplifier 26 connected to the output offorward amplifier 18 through directional couplers 20 and 28. The signalreceived from the forward amplifier is amplified by bridger amplifier 26and then split into four signals by directional couplers 29, 30 and 31.Those four signals are coupled to separate feeder lines along outputlines 33, 34, 35, and 36. The output of forward amplifier 18 is alsoconnected, through directional couplers 20 and 28, to an automatic gaincontrol (AGC) module 38 provided with circuitry for automatic control offorward amplifier gain and slope, characteristics which will beexplained further below. AGC module 38 supplies control signals toforward amplifier 18 along the GAIN and SLOPE output lines for controlof forward amplifier gain and slope. Elementary components suitable forthe trunk station functions just described are commercially available,and are available as individual plug-in modules from Texscan El Paso,1440 Goodyear Drive, El Paso, Tex. 79936. Trunk station 10 also includescircuitry and controls (not shown) for manual control of gain and slope.Forward amplifier 18 and bridger amplifier 26 are both feedforwardamplifiers of similar architecture, and the circuitry described hereinas to the forward amplifier is suitable with minor modifications for usein a bridger amplifier. Also, reverse amplifier 24 can be implemented asa feedforward amplifier applying the teachings of the present invention.Such an implementation is desirable for mid-split systems, in which anincreased number of channels are carried in the return segment fortransmission of programs from multiple local origination sources, andthe bandwidth of the forward segment is correspondingly reduced. Trunkstation 10 is preferably housed in a hinged case (not shown) for easyaccess, and each amplifier in the station is a plug-in module with ahousing having both a top and bottom cover (not shown).

Trunk station 10, including the feedforward amplifiers containedtherein, is powered by a single -24 volt DC power supply. Present-dayhigh-gain hybrids have significant frequency response ripple when usedin negative power supply systems. The use of medium-gain hybrids in afeedforward topology allows use of a negative-voltage power supply in afeedforward device. As used herein, high-gain refers to approximately34-38 dB, medium-gain refers to approximately 18-24 dB, and low-gainrefers to approximately 12-14 dB. Also as used herein, a hybrid deviceis one containing thick-film construction in addition to one or morediscrete components. A conventional hybrid amplifier, for example,includes integrated transistors, thick-film resistors, and discretecomponents including surface-mount chip capacitors and miniaturetoroidal transformers, the transformers being used for input and outputcoupling.

FIG. 2 depicts in block diagram form the circuit components contained ina conventional forward amplifier. A signal applied to the input of theamplifier is fed to a pad 40 and equalizer 42 for broadband adjustmentof the signal level prior to amplification by input hybrid amplifier 44.In those installations not spaced at a typical value because ofgeographical or other restraints, the pad and equalizer are selected tomaintain operation of the attenuator stage 46 and slope control circuit50 at their electrical midrange positions in a nominal ambienttemperature. The equalizer corrects for tilt in the incoming signalspectrum caused by frequency-dependent cable attenuationcharacteristics. Adjustments are made in steps. Equalization isperformed at this stage because the output of the first amplifier,hybrid 44, is desirably flat across the band of interest. The gain ofhybrid amplifier 44 is commonly one of three values--14, 18 or 22 dB--toprovide sufficient trunk amplifier gain to support spacing of 22, 26 and30 dB, respectively. In practice, this hybrid is selected on the basisof gain required for a desired spacing. PG,14

The amplified signal is fed to an attenuator stage 46 that works inconcert with a slope control circuit 50 to maintain a constant signallevel for output amplifier 52 to amplify. Whereas equalizer 42 adjustslevels in stepwise fashion, slope circuit 50 provides continous levelchanges. Otherwise, the two circuits are functionally similar. Amplifier52 is a feedforward stage which may be of discrete or hybrid design.Thus, as shown in FIG. 2, a conventional feedforward amplifier containsa hybrid stage 44, an attenuator section 46 for gain adjustment, a slopesection 50 for cable equalization over temperature, and a feedforwardstage 52. Systems are specified by the spacing at the highest frequency.Typical values for these stages in a 22 dB-spaced system, with theattenuator and slope controls set at midrange, are 14 dB gain, 6 dBloss, 4 dB loss and 21 dB gain, respectively.

The control signal on the GAIN output line of AGC module 38 (FIG. 1) iscoupled to input 47 of attenuator section 46 for attenuation control.Similarly, the control signal on the SLOPE output line of AGC module 38is coupled to input 51 of slope section 50 for slope control. It isdesirable for the controls 46 and 50 to be operated at their electricalmidranges at a nominal ambient temperature to allow sufficient signallevel adjustment to compensate for changes in attenuation as the cablespan preceding the station expands or contracts due to temperature. Bymaintaining a constant signal level for the output hybrid to amplify,the distortion performance of the amplifier module remains fairlyuniform over temperature. In long cascades of trunk amplifiers, responsesignatures may develop because of a common response anomaly. Forexample, the frequency response of a hybrid amplifier typically rollsoff at the band edges due to bandwidth and internal ferrite devices.Additionally, peaks and valleys exist in the midband frequency response.Other devices in the system, for example, connectors and filters, canintroduce common response aberrations. Such aberrations present aproblem when they are common to a number of cascaded devices. Signaturecontrol circuit 48 introduces a flat loss with appropriate valleys andpeaks to reduce the impact of these common response aberrations.

The preferred embodiment of a feedforward amplifier for use as forwardamplifier 18 in trunk station 10 is illustrated in block diagram form inFIG. 3. Gain and slope control are combined into the first amplifierstage 144, which has input lines 147 and 151 for control of gain andslope, respectively. Pad 140, equalizer 142, and signature controlcircuit 148 are the same as their corresponding circuits in the forwardamplifier of FIG. 2. Output stage 152 is a feedforward amplifier stagewhich will be described in more detail with reference to FIG. 6. Toenable upgrades of 220 MHz systems up to 450 MHz, and to allow for gainadjustment for different trunk station spacings, amplifier 18 furtherincludes a booster amplifier 154, a plug-in printed circuit (pc) cardpreferably available in three versions corresponding to gain values of0, 4 and 8 dB, the 0 dB card consisting simply of a jumper. For example,a 22 dB-spaced system simply requires a jumper in the position of thebooster amplifier. The booster amplifier is selected to fit the desiredgain criterion.

Booster amp 154 is preferably a push-pull amplifier to obtain goodsecond order distortion performance. It is mounted on a plug-inminiature printed circuit board which mounts into a mother board inforward amplifier 18. A number of other stages within the forwardamplifier are configured as plug-in circuit boards, including activegain and slope control circuit 144, signature control circuit 148, and,in feedforward amplifier 152, a hybrid trim amplifier (to be describedbelow) as well as separate first and second stages of the erroramplifier. The error amplifier is configured as two plug-in gain stageseach having two cascaded stages. Additionally, the bottom cover of theamplifier includes means for quickly connecting it to the mother boardof the amplifier module. This construction enhances producibility of thetrunk amplifier by enabling module-by-module testing andtroubleshooting.

Some of the advantages of the present invention may be appreciated byconsidering the method of achieving the adjustable cable compensation incircuits 46 and 50 (FIG. 2). Most amplifiers of this type provide acable adjustment of +/-4dB from the nominal operating spacing of thecable, which for many CATV systems is approximately 22 dB at the highestoperating frequency. With the gain and slope controls set at midrange,the insertion loss of attenuator 46 is typically 6 dB flat, and slopecircuit 50 has typically 4 dB loss at the highest operating frequency,with the loss following the inverse of cable response. To achieve cabletracking over temperature, the gain and slope controls work in concert,as will now be described. It will be understood that insertion lossrefers to high-frequency insertion loss. FIG. 4A illustrates the losscharacteristic of slope circuit 50 with three curves, labeled 1, 2 and3, which correspond with low, nominal and high ambient temperatures,respectively. Curve 1 has a response characteristic to compensate for a4 dB decrease in station spacing caused by cable shrinkage at lowtemperature. The attenuator 46 works in concert with slope circuit 50,adjusting in a continuous fashion from 6 dB flat loss at nominal to 10dB flat loss at low temperature. Similarly, curve 3 has a responsecharacteristic to compensate for a 4 dB increase in spacing caused bycable expansion at high temperature, and attenuator 46 adjusts to 2 dBflat loss at high temperature. The loss characteristic of FIG. 4A iscommonly referred to as a "high-end pivot" because the slope can beadjusted between curves by "pivoting" the response characteristic abouta common point at the high-frequency end of the curves. This interstageloss must be overcome by the input hybrid 44. To ensure that the inputhybrid does not dominate the distortion performance of the amplifier,considerable current must flow in the transistors associated with theclass A cascode topology. Third order distortions are minimized byconsiderable current flow, and second order distortions are minimized bya balanced topology, which doubles the current consumption of thecascode topology. The interstage loss also increases the gain requiredof the input stage.

The present invention provides a means for achieving cable adjustmentwhile providing gain at reduced current consumption, as will bedescribed with reference to FIGS. 4A-4E. FIG. 4B is a schematicrepresentation of gain and slope control amplifier 144. A common-emitteramplifier stage 60 has a feedback network 62 that providescable-equalized gain. FIG. 4C illustrates the loss characteristic ofcable-equivalent network 62. In this figure, curve 2 corresponds withnominal ambient temperature, while high-temperature response is shown incurve 3 and low-temperature response in curve 1. The high-temperatureand low-temperature losses at the high-frequency end are each 4 dB awayfrom nominal, for a total of 8 dB of cable tracking, the same as FIG.4A. However, FIG. 4C illustrates a "low-end pivot" loss characteristic.FIG. 4D shows the corresponding low-end pivot gain characteristic forthe slope control circuit in gain and slope control amplifier 144. Thecurves of FIG. 4C approximate the inverse square-law response associatedwith copper losses in coaxial cable. Coaxial cable also has adialelectric loss which is linear with frequency, but, for the half-inchto one-inch cables which are used for CATV trunk lines, that loss isnegligible compared to the copper loss for frequencies below 600 MHz.Gain is controlled by a control signal applied to control line 147, andslope control is effected through control line 151. Although controlline 147 is depicted as the control mechanism of a variable resistanceelement 64, variable resistance element 64 is implemented with a PINdiode which receives its control input on line 147. One embodiment of acable-equivalent network 62, shown in FIG. 4E, is a network consistingof a capacitor 68, inductor 69, and a variable resistance 70 which isalso implemented with a PIN diode. The signal on control line 151determines the resistance of PIN diode 70. The gain of stage 144 isapproximately 10 dB, which approximately equals the combined gain of a14 dB input hybrid 44 and a passive slope circuit 50 havingapproximately 4 dB of loss. The reduced gain of the network comprising60 and 62 and associated components allows lower operating bias as thesignal level is lower.

Active gain and slope circuit 144 replaces two passive circuits thatcontribute significantly to the module's interstage losses. The lossthat is incurred with the passive topology must be overcome with theappropriate gain device at the input of the amplifier. This is achievedat the expense of distortion contribution from the input device. Byeliminating as much interstage loss as is practical, the gain and signallevels can be reduced accordingly. Lower signal levels will require lessdistortion capability from the input device. Hence, the input device canbe a discrete amplifier stage, which has a significantly lower currentconsumption than a conventional hybrid amplifier. Accordingly, amplifier144 is a discrete stage. An improvement in second order distortionperformance can be achieved by employing a push-pull topology for thisstage. Such a topology has current consumption somewhat greater thanthat for the single-ended topology shown in FIG. 4B but still allows asignificantly reduced current consumption over a conventional hybridamplifier. The savings in current consumption contribute significantlyto the overall reduction in current consumption of the feedforwardamplifier.

The configuration and operation of feedforward output stage 152 offeedforward amplifier 18 will now be described. The basic feedforwardtopology has been described in a number of patents and articles, forexample, U.S. Pat. No. 4,028,634 to Tentarelli, issued June 7, 1977;U.S. Pat. No. 4,130,807 to Hall et al., issued Dec. 19, 1978; and"Feedforward" by Charles Evans, in Communications/Engineering Digest,April, 1977. It is also illustrated in block diagram form in FIG. 5. Asshown there, a feedforward amplifier includes two parallel signal paths,a main path and a feedforward path, connected to common input and outputterminals through directional couplers 75 and 76. The main path includesa main amplifier 80, a delay line 84, and a directional coupler 82interconnecting the main amplifier and the delay line. Delay line 86,directional coupler 88 and error amplifier 90 comprise the feedforwardpath. An attenuation network 92 interconnects the first and seconddirectional couplers. In this device, main amplifier 80 and erroramplifier 90 are both 34 dB gain hybrid amplifiers having similaroperating characteristics. All couplers are 10 dB couplers, which haveapproximately 10 dB loss at the coupled port and approximately 1 dBthrough loss. An incoming signal is input to directional coupler 75which couples the signal to the main amplifier 80. The signal isamplified and passes to the output through coupler 82, delay line 84 andcoupler 76. A portion of the input signal is fed forward and arrives atthe input to the error amplifier 90 by way of delay line 86 and coupler88. Simultaneously, a portion of the amplified signal from mainamplifier 80 is coupled, padded, and injected by coupler 82, 7 dBattenuation pad 92, and coupler 88 respectively, to effectively cancelthe signal arriving at error amplifier 90 from the input port. Thesignal present at the error amplifier contains the distortion productsand noise power developed by the main amplifier. The error amplifieramplifies the distortion and noise generated by the main amplifier andinjects this signal into the output circuit, thereby cancelling to ahigh degree the distortion and noise associated with the main amplifier.The state of the art allows approximately a 20 dB improvement in thirdorder distortions such as composite triple beat and crossmodulation.

A significant disadvantage with the above process as it pertains to CATVamplifiers is the considerable increase in power consumption over thatof a comparable hybrid amplifier module. A high gain hybrid draws 340 mAof current which is approximately 120 mA more than a medium gain hybrid.It will be understood that 24 volts DC is the standard for CATVamplifiers, most systems operating from +24 VDC while others use -24VDC, and that accordingly it is appropriate to compare power consumptionin terms of current drain. The combination of main and error amplifiersalone in the feedforward topology draws 680 mA versus the typical 410 mAof a conventional, non-feedforward amplifier module. Referring again toFIG. 2, including the current consumption of the input hybrid 44, theaggregate module current consumption for a feedforward trunk module isapproximately 900 mA, more than double the current consumption of thenon-feedforward module.

A means to reduce the current consumption of the prior art feedforwardtopology is shown in FIG. 6. A medium-gain hybrid is utilized for mainamplifier 180. As seen in the drawing, main amplifier 180 has gain ofapproximately 22 dB. Each of the 10 dB couplers 176 and 188 hasapproximately 1 dB of through loss, while the through losses for the 8dB coupler 175 and 16 dB coupler 182 are 1.5 dB and 0.5 dB,respectively. The delay lines 184 and 186 are preferably miniaturecoaxial cable on the order of one-eighth to one-sixteenth of an inchwith approximately 0.3 dB loss each, as will be explained later. Thus,the gain of the feedforward stage is calculated as follows:

    GAIN=(22-1.5-0.5-0.3-1) dB=18.7 dB

For ease of comparison with the amplifier of FIG. 5, the gain is roundedoff to 19 dB in the drawing. Thus, the low-power feedforward amplifierof FIG. 6 has approximately the same gain as that of FIG. 5, yet thepower consumption is substantially reduced and the distortionperformance is still competitive. Error amplifier 190 is a four-stagecommon-emitter amplifier comprising amplifier stages 190a, 190b, 190cand 190d. This four-stage amplifier achieves approximately 38 dB gainwhile drawing only 100 mA of current. The circuit configuration whichprovides reduced current consumption in the error amplifier will bedescribed shortly. The justification for reduced current consumption ofthis stage is that the signal being amplified has a very low averagepower level, hence the requirement for high bias currents has beenalleviated.

Thus, the lower power feedforward topology replaces the presentstate-of-the-art topology, allowing gain and distortion capabilitycompetitive with the present topology, but at a current consumption thatis one-half that of the present topology. A medium-gain conventionalhybrid is utilized as the main amplifier, with the RF attributes of thishybrid being compensated by a discrete preamplifier, as will bedescribed with reference to FIG. 9. The error amplifier comprises amultistage discrete amplifier. With adequate loop cancellation, theeffects of distortion being introduced by the error amplifier are ofsecondary importance, allowing a significant reduction in currentconsumption of this stage.

Two stages of error amplifier 190 are shown schematically in FIG. 7B,with base circuit bias resistors excluded for ease of illustration.Those skilled in the art will understand that the quiescent collectorcurrent of a transistor is a direct function of the quiescent basecurrent, and that collector bias currents may accordingly be separatelyanalyzed for purposes such as those described herein. A common practicein amplifier design is the use of bias dropping resistors when operatingsingle-ended stages from a supply voltage higher than the quiescent andpeak signal voltage swing. This is illustrated by FIG. 7A, whichschematically depicts two cascaded stages of a multistage amplifierwhich does not employ current sharing techniques. A low-valued resistor96 is provided for gain stabilization and impedance matching purposes.An additional bias dropping resistor 97 in series between resistor 96and the -V power supply dissipates power without contributing to circuitfunction. The power dissipated is approximately the same as thatdissipated by the amplifier stage 98. With adequate decoupling of theemitter resistor 96, two gain stages 98 and 99 may share the samebiasing current, thereby reducing the current consumption by a factor oftwo. This is shown in FIG. 7B, wherein the voltage dropping resistors 97are eliminated from the circuit, the connection of both stages 98 and 99to the -V power supply is made through a low-valued resistor 100.Resistor 100 provides current limiting rather than bias dropping, andaccordingly is a much smaller value than resistor 97. Due to biascurrent sharing, the net current consumption in error amplifier 190 isreduced from 200 mA to 100 mA.

Current sharing techniques are applied throughout the amplifier toachieve aggregate module current consumption that rivals that ofconventional hybrid amplifiers. As alluded to above, this techniquegenerally involves elimination of a voltage dropping resistor in theemitter bias network of one stage in favor of a voltage droppingresistor in another gain stage operating at quiescent levels the same asthe emitter bias resistor. Both stages are in series with respect to theDC bias, and they may or may not be associated in terms of RF signalflow. As described above, this technique is employed in the erroramplifier, wherein the stages are cascaded, to cut the currentconsumption of this stage by one-half. It is also applied to twoamplifier stages which are separate from each other in terms of RFsignal flow, as will be described later with reference to FIG. 10.Furthermore, it will be appreciated that this technique is equallyapplicable to current sharing between one stage of the error amplifierand one or more amplifiers not functionally related to the erroramplifier, as well as to other applications.

The component density in the amplifier housing was found to beprohibitive for placement of coaxial cable as the delay mechanism in thefeedforward topology. The size of the overall trunk station is limitedby space restrictions imposed in some localities, and space within thetrunk station and, accordingly, within the individual amplifier modulesis at a premium in many other situations because of retrofitrequirements. In view of space limitations within the amplifier housing,the delay lines in one embodiment are fabricated using striplinetransmission line mounted on the bottom cover of the amplifier housing,as seen in FIG. 8. The ground plane associated with striplineeffectively contains the RF signals within the amplifier housing. Thesignature control circuit 148 has adjustments that may be accessed fromthe top of the amplifier module, through holes in the top cover. Thebottom cover's ground integrity is not broken. As space for supportingcircuitry in the amplifier module is at a premium, as stated, a novellocation for the placement of delay lines 184 and 186 required in thefeedforward topology has been found to be the bottom cover of theamplifier module. The stripline transmission line construction of FIG. 8achieves the delay required while maintaining acceptable shielding bothfor the amplifier module and for the delay lines.

The embodiment shown in FIG. 6 uses miniature coaxial cable of theaforementioned type to effect the delay lines. Miniature coax hasinsertion loss characteristics far superior to stripline, microstrip andlumped-element delay networks and, in that regard, is preferable overthose forms of delay line. For a delay line exhibiting a 3 nanoseconddelay, which is the approximate delay required at the operatingfrequency of a CATV amplifier module such as that disclosed herein, theinsertion loss of miniature coax is on the order of 0.3-0.4 dB. Byreducing the loss of the output leg seen by main amplifier 180, thedistortion will be reduced accordingly.

FIG. 9 is a block diagram of a low-power feedforward amplifier havingcertain advantages over the feedforward amplifier shown in FIG. 6. Theamplifier of FIG. 9 includes input and output directional couplers 275(6 dB) and 276 (10 dB), a main path including main amplifier 280, 10 dBdirectional coupler 282 and delay line 284, a feedforward path includinga delay line 286, 10 dB directional coupler 288, and a four-stage erroramplifier 290, and a 5.3 dB attenuator pad 292 in the crossover positionbetween the main and feedforward paths. Both delay lines are constructedof miniature coax of the aforementioned type. The four stages of erroramplifier 290 have gain of 9.5 dB each, or 38 dB total gain. A 5 dB padis inserted between stages 290b and 290c to achieve the proper gainlevel for cancellation. Additionally, this amplifier includes apreamplifier 278, with nominal gain of 10 dB, which is used as a hybridtrim amplifier, that is, an amplifier used to trim the loop performanceto compensate for varying characteristics of the hybrid amplifier usedas main amplifier 280. Present-day hybrids come in gain ranges thatdeviate as much as 1 dB and have tilt of as much as 1.5 dB. Hybrid trimamplifier 278 provides control over the response of the cascaded gainsection consisting of amplifiers 278 and 280. Response trimming in thismanner enhances cancellation of distortion and noise products generatedby the main amplifier and thereby alleviates yield problems associatedwith hybrid amplifier gain and tilt specifications. Trim amplifier 278also includes thermal compensation circuitry to compensate for changesin hybrid amplifier 280 over temperature. In general, the gain of thehybrid amplifier is approximately inversely proportional to temperature.Thus, for optimum control of cancellation over temperature, some meansfor offsetting the hybrid gain changes over temperature is desirable.Hybrid trim amplifier 278 includes a thermistor network which adjuststhe gain of the trim amplifier in an approximately direct proportionwith temperature so as to maintain a substantially constant gain overtemperature for the cascaded stages 278 and 280. Temperaturecompensation networks having the above-described desired characteristicsare well known and therefore need no further description.

Another advantage of the embodiment shown in FIG. 9 is improved noisefigure. The noise figure of a feedforward amplifier is the sum of thelosses through the input directional coupler and all elements in thefeedforward path, i.e., directional couplers 275 and 288, delay line 286and error amplifier 290. Directional coupler 275 is connected with itsthrough port rather than its coupled port connected to delay line 286 sothat its through loss is incurred in the feedforward path instead of themain path. The overall noise figure of the amplifier is reduced andthereby improved because the coupler through loss is approximately 2 dB,substantially less than the 6 dB loss through the coupled port.

The gain of the feedforward stage of FIG. 9 is calculated as follows:

    GAIN=(10+18-6-1-0.3-1) dB=19.7 dB

As before, the gain is nominally indicated as 19 dB.

Referring now to FIG. 10, current sharing between isolated amplifiers isillustrated schematically in an exemplary form in which active gain andslope control circuit 144 shares bias current with hybrid trim amplifier278. Gain and slope control amplifier 144 is modified by removal of thebias dropping resistor 63 (FIG. 4B), emitter resistor 61 being connecteddirectly to the tapped coil in the collector circuit of transistoramplifier 278. Amplifier 278 includes an emitter resistor 293, forimpedance matching and gain stabilization, and a bias dropping resistor294 connecting emitter resistor 293 to the -V supply terminal. Asillustrated by FIG. 10, current consumption in the trunk amplifiermodule is reduced by applying the current sharing technique not only tocascade-connected amplifiers but also to amplifiers which are isolatedwith respect to the RF signals being amplified.

FIG. 10 also schematically illustrates the feedback network whichprovides the frequency response trimming to compensate for varying gainand tilt characteristics of hybrid amplifiers used as the main amplifierin the feedforward loop. DC blocking capacitor 296, variable inductance297 and variable resistance 298 enable adjustment of both gain and tiltof the hybrid trim amplifier stage, thereby providing adjustment of thecascaded amplifier stages 278 and 280 in the main path of thefeedforward amplifier of FIG. 9. The variable resistance 298 is apotentiometer which need be set only once during amplifier alignment.Variable inductance 297 is similarly set only once, and it is preferablyan air-dielectric inductor.

FIG. 11 illustrates, partly schematically and partly in block diagramform, a two-stage active gain and slope control circuit. This controlcircuit has a cascode configuration. The input to the circuit issupplied to a common-emitter stage 310 the output of which is connectedthrough a cable-equivalent network 312 to a common-base amplifier 314.Cable-equivalent network 312 includes a bias shunting path, preferably achoke, for common conduction of bias current through stages 310 and 314.In other respects this network is similar to that shown in FIG. 4E. Gainis controlled by a control signal applied to control line 147 leading tothe PIN diode connected to the emitter of stage 310, and slope controlis effected through control line 151 leading to cable-equivalent network312. This circuit configuration provides a somewhat improved distortionperformance over that of a single-stage amplifier. The noise figure isimproved because the gain of stage 310 is higher with cable equivalentnetwork 312 located between the output of stage 310 and the input ofstage 314 instead of in the feedback path around stage 310. The feedbackpath is from the collector tap of stage 314 to the base of stage 310,and the feedback network is preferably a series RLC network. Animprovement in second order distortion performance can be achieved byemploying a push-pull topology for this stage. Such a topology hascurrent consumption somewhat greater than that for the single-endedtopology shown in FIG. 11 but still allows a significantly reducedcurrent consumption over a conventional hybrid amplifier.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

I claim:
 1. A low-power feedforward amplifier, comprising:(a) a mainamplifier; (b) means for comparing a sample of an input signal suppliedto said main amplifier with a sample of an output signal from said mainamplifier, said comparing means producing an error signal proportionalto the difference between said signal samples; and (c) means foramplifying said error signal and combining the amplified error signalwith the main amplifier output signal in opposite phase, wherebydistortion and noise products generated by said main amplifier aresubstantially cancelled, said amplifying and combining means including ahigh-gain error amplifier having power consumption below that requiredby a conventional medium-gain hybrid amplifier.
 2. The low-powerfeedforward amplifier of claim 1 wherein said main amplifier is amedium-gain amplifier.
 3. The low-power feedforward amplifier of claim 2wherein said main amplifier is a hybrid amplifier and said erroramplifier is a discrete amplifier.
 4. The low-power feedforwardamplifier of claim 3 further comprising:(d) an amplifier housing havinga cover, wherein said comparing means and said amplifying and combiningmeans each include a delay line constructed of stripline and mounted onsaid cover.
 5. The low-power feedforward amplifier of claim 4 furthercomprising:(e) means for energizing said main amplifier and said erroramplifier from a negative-voltage power supply.
 6. The low-powerfeedforward amplifier of claim 3 further comprising:(d) an amplifierhousing having a cover, wherein said comparing means and said amplifyingand combining means each include a delay line constructed of miniaturecoaxial cable and mounted on said cover.
 7. The low-power feedforwardamplifier of claim 6 further comprising:(e) means for energizing saidmain amplifier and said error amplifier from a negative-voltage powersupply.
 8. The low-power feedforward amplifier of claim 1 wherein saidmain amplifier is a medium-gain amplifier and said error amplifier is ahigh-gain amplifier.
 9. The low-power feedforward amplifier of claim 8wherein said main amplifier is a hybrid amplifier and said erroramplifier is a discrete amplifier.
 10. The low-power feedforwardamplifier of claim 8 further comprising:(e) means for energizing saidmain amplifier and said error amplifier form a negative-voltage powersupply.
 11. A low-power feedforward amplifier, comprising:(a) a mainpath and a feedforward path, said main path including(1) a mainamplifier; (2) a first delay line; and (3) a first directional couplerdirectly interconnecting said main amplifier and said first delay line;said feedforward path including(1) a second delay line; (2) a high-gainerror amplifier having power consumption below that required by aconventional medium-gain hybrid amplifier; and (3) a second directionalcoupler directly interconnecting said second delay line and said erroramplifier; (b) an input directional coupler directly interconnecting asignal input of said feedforward amplifier, said main amplifier and saidsecond delay line; (c) an output directional coupler directlyinterconnecting said first delay line, said error amplifier, and asignal output of said feedforward amplifier; and (d) an attenuationnetwork directly interconnecting said first and second directionalcouplers.
 12. The low-power feedforward amplifier of claim 11 furthercomprising:(e) an amplifier housing having a cover, wherein said firstand second delay lines are constructed of stripline and mounted on saidcover.
 13. The low-power feedforward amplifier of claim 11 furthercomprising:(e) an amplifier housing having a cover, wherein saidcomparing means and said amplifying and combining means each include adelay line constructed of miniature coaxial cable and mounted on saidcover.